Geometry for pulsed acoustic measurements of particle size

ABSTRACT

An apparatus for making pulsed acoustic measurements to determine particle size in a sample ( 1 ) such as a colloid. The apparatus includes two electrodes ( 2 ) between which an electric voltage is applied to the sample. The voltage is applied in short pulses. The applied voltage generates an acoustic signal, such as a sound wave ( 5 ), in the sample which is detected with a transducer ( 3 ) after the wave ( 5 ) has passed through a delay element ( 4 ). The delay element ( 4 ) is used to introduce enough of a time delay between the application of the voltage pulse and sound wave ( 5 ) reaching the transducer so that the received wave&#39;s signal can be isolated from any signal generated in the transducer ( 3 ) by the applied voltage. The delay element ( 4 ) and the transducer ( 3 ) are arranged into a geometry, or, alternatively, the delay element ( 4 ) is shaped, so that any reflections of the sound wave ( 5 ) from the sidewalls of the delay element ( 4 ) are deflected away from the transducer ( 3 ). This reduces interference, particularly at low frequencies.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for use inmaking pulsed acoustic measurements, particularly ultrasonicmeasurements.

BACKGROUND OF THE INVENTION

Measurements of the response of a material to a pulsed acoustic signalare used in a variety of applications. Whilst it is more broadlyapplicable, the present invention will be predominantly described inrelation to the field of electroacoustic measurements.

U.S. Pat. Nos. 5,059,909, 5,245,290 and 5,616,872 describe the theoryand application of electroacoustic measurements in determining particlesize and zeta potential within colloids. They do this by makingelectroacoustic measurements, which involve the application of ahigh-frequency (typically MHz) alternating voltage across the colloid.This voltage generates sound waves which are measured by a transducer.From these measurements of sound waves as a function of frequency theparticle size and zeta potential of the colloidal particles can bedetermined. The applicant manufactures commercially a device, marketedas the AcoustoSizer, which measures size and zeta potential using thismethod.

The AcoustoSizer measurement procedure is illustrated in FIG. 1.

The applied voltage generates sound waves 5. Although these sound waves5 are generated by the colloid 1, they appear to come from theelectrodes 2 (for an explanation see §3 of the paper by O'Brien, Cannonand Rowlands in “Journal of Colloid Interface Science”, No. 173,p406-418 (1995)). Thus two sound wave beams are generated; one from eachelectrode. The signal from the right hand electrode in FIG. 1 is the oneused for determining particle size and zeta potential in theAcoustoSizer. When that signal reaches the transducer 3, it generates asmall voltage. That voltage is very much smaller than the drivingvoltage originally applied across the colloid (by a factor of 100,000 orso). This creates a measurement problem, for it is very difficult tocompletely isolate the leads of the transducer from the large appliedvoltage, and as a result there will usually be a component of thetransducer signal due to “cross talk” from the applied voltage. In orderto remove this cross talk the AcoustoSizer uses a combination of pulsedsignals and a delay line 4. Instead of applying a continuous sinusoidalvoltage across the colloid 1, the AcoustoSizer applies a pulsedsinusoid, which is simply a sinusoid of limited duration—typically a fewmicroseconds. The glass block 4 in the above diagram is the delayline—its function is to introduce a delay between the application of thevoltage to the colloid 1 and the sound wave 5 reaching the transducer.When the sound wave 5 has arrived at the transducer 3, the appliedvoltage has been turned off, and there is no longer any cross talk. Theelectrode spacing and the pulse width are chosen so that the sound wavepulse from the second electrode reaches the transducer 3 after the soundwave pulse from the closest electrode has finished. Thus the pulses donot overlap in the transducer 3 and it is possible to gate off andmeasure the signal from the closest electrode.

One limitation of the AcoustoSizer approach to measurement is that itworks only if the sound wavelength 5 in the glass block 4 is muchsmaller than the cross sectional dimensions of the glass block 4. Thisrestricts the device to frequencies above 1 MHz. At lower frequenciesthe beam spreads out and is reflected from the side walls of the glassblock 4. The reflected signals interfere with the signal from theclosest electrode and instead of getting two distinct signals from thetwo electrodes, we obtain one smeared out signal. Thus it is notpossible to gate off and measure the signal from the closest electrode.

It is desirable that the delay line be operable at lower frequencies.Larger particles have greater inertia, and so a lower frequency signalwill produce larger movements and hence better measurements forcolloidal particles which include larger particles. The use of lowerfrequency measurements in other applications is similarly constrained.

One known low frequency colloid analyser is described in U.S. Pat. No.4,907,453. A piezoelectric transducer produces a continuous, lowfrequency, low power acoustic signal. This signal is propagated througha colloid sample towards a receiver. However, the use of an appliedvoltage to generate an acoustic signal in the colloid is not disclosedand hence the difficulties associated with “cross talk” and the use of adelay line are not considered.

A known device for measuring particle size distribution and zetapotential is described in U.S. Pat. No. 6,109,098. A piezoelectrictransducer produces pulsed acoustic signals. These signals arepropagated through a colloid sample towards a receiver via quartz delayrods. The device suffers from beam spreading at low frequencies andhence can only be reliably operated above 1 MHz.

A known method of detecting the onset of colloid formation is describedin WO 00/74839. An oscillating electric field is applied to a sample andan acoustic signal is generated. This acoustic signal propagates throughan acoustic delay line to a detector. This method uses the standard typeof delay line. The acoustic signal passes down a rod of fixed crosssection and no consideration is given towards focusing the signal beam.

It is an object of the present invention to provide a method for pulsedacoustic measurements, and an apparatus for such measurements, which isoperable with wavelengths which are not much smaller than the dimensionsof the delay line.

SUMMARY OF THE INVENTION

Broadly, the present invention provides for the use of a delay line, inwhich the geometry is altered so as to minimise sidewall reflections andtheir adverse effects on measurement, particularly at the point wherethe transducer is located. A preferred form uses curved, for examplecircular, geometry to focus the acoustic waves away from the wall.

According to one aspect, the present invention provides an apparatus formaking pulsed acoustic measurements, including means for generating anacoustic response in a sample, a delay element, and a transducer fordetecting the acoustic response,

characterised in that the delay element and transducer are arranged tohave a geometry so as to reduce the effect of sidewall reflectionsrelative to a similarly sized rectangular delay element.

Preferably, the delay element includes interfaces with a curvedgeometry, so as to deflect the sidewall reflections away from the siteof the transducer. In one form, the transducer may be located on theopposite side of a curved element to the source of the acoustic signal.The acoustic signal may be the acoustic response to an electrical pulseacross a sample of a colloid.

According to another aspect, the present invention provides an apparatusfor making pulsed acoustic measurements, including means for generatingan acoustic response in a sample, a delay element, and a transducer fordetecting the acoustic response,

characterised in that the delay element is shaped so that the walls ofthe delay element are oriented so that sidewall reflections do notsubstantially propagate to the transducer.

BRIEF DESCRIPTION OF DRAWINGS

Illustrative embodiments of the present invention will now be describedwith reference to the accompanying figures, in which:

FIG. 1 illustrates schematically the prior art approach to measurement;

FIG. 2 illustrates a first embodiment of the present invention;

FIG. 3 illustrates a second embodiment of the present invention;

FIG. 4 illustrates a third embodiment of the present invention;

FIG. 5 illustrates a typical transducer signal using an embodiment ofthe present invention; and

FIG. 6 illustrates a typical transducer signal using the background artdevice.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention will be described in relation to several possibleembodiments of the present invention. It will be appreciated that thereare many possible arrangements which can achieve the objects of theinvention, and those presented are merely illustrative.

The inventor experimented with a number of geometries for the glassblock 4. Other rectangular shapes were tried, as well as cylindricaldelay lines—but they all showed substantial sidewall reflections at thetransducer 3. The only way to allow for lower operating frequencies withdelay lines shaped this way is to increase the cross sectionaldimensions of the block. This in turn leads to the dimensions of theinstrument being increased, which is undesirable from a cost and spaceperspective.

FIG. 2 illustrates a design that uses circular geometry to focus thebeams away from the wall. This enables clean electroacoustic pulses tobe detected without sidewall reflections smearing the signal.

When the voltage is applied across the electrodes 12, sound waves aregenerated as in the AcoustoSizer, but because of the circular geometrythere is a tendency for the sound waves to be focussed as they movetowards the centre of the solid cylinder 14. There will still be somespreading out of the wave, but waves that do hit the sides of the rodwill tend to get reflected back, rather than being directed to thetransducer 13. As a result the signals from the first electrode is notoverlapped by any other signals, and we can make our measurement, eventhough the wavelength is not small compared to the rod diameter.

FIGS. 5 and 6 illustrate the advantage of the cylindrical geometry. FIG.5 illustrates the transducer signal at 300 kHz using the device geometryshown in FIG. 2. In this case the diameter d of the solid cylinder 4,which is made of plastic, is 30 mm. The sound wavelength λ in theplastic is 7.3 mm at this frequency, and thus the ratio d/λ isapproximately 4. FIG. 6 shows the waveform obtained using a rectangularglass block at the same frequency. The cross sectional dimensions of theblock have been chosen to give the same dλ ratio. In this example, theblock has a square cross sectional area having sides of approximately 70mm. Clearly the gated waveform in FIG. 6 is much more smeared out; andit cannot be readily used for measurement as it cannot be easily gated.

FIG. 2 shows only one possible implementation. FIG. 3 illustratesanother possible geometry including an annular device with a concentriccylinder geometry. The electrodes 22 are cylindrical and containsample 1. This has the advantage that the beam from the colloid 1 isfocused onto the central transducer 23, and in the focussing it will beamplified. The disadvantage compared to the design in FIG. 2 is that ithas to be larger in order to have same delay between the applied pulseand the sound wave arriving at the transducer 23.

FIG. 4 illustrates a further implementation of the present invention. Inthis device the colloid 1 is contained in a central cup and thetransducer 33 is on the outer part of the design. Electrodes 32 arearranged to point towards transducer 33. So, the beam from the colloid 1is spread out, rather than being focussed. The sidewall reflections arereduced because of the divergence of the sidewalls in delay element 34.The advantage of this device is that it would be easier to stir thecolloid 1 in this cup than in the annular geometries above.

The examples provided have centered on the problem of measuring theelectroacoustic signal using the pulse-delay line technique. There areother measurements that are also made by pulse-delay line techniques.These include ultrasonic attenuation, speed of sound and acousticimpedance of a material. In these measurements the voltage is appliedacross a transducer rather than the colloid. A sound wave is generatedand that sound wave is then measured after passing through or beingreflected by the material. Although the measurement is different theprinciple is the same: by using an altered geometry, it is possible touse lower frequencies for a given size of apparatus.

It will be appreciated that additions and variations are possible withinthe scope of the present invention, without departing from the generalinventive concept.

1. An apparatus for making pulsed acoustic measurements, including means for generating an acoustic response in a sample, a delay element, and a transducer for detecting the acoustic response, the transducer being positioned so as to receive said response through said delay element, characterised in that the delay element and transducer are arranged to have a geometry so as to reduce the effect of sidewall reflections relative to a similarly sized rectangular block delay element.
 2. An apparatus according to claim 1, wherein the delay element includes interfaces with a curved geometry, so as to deflect the sidewall reflections away from the site of the transducer.
 3. An apparatus according to claim 2, wherein the transducer is located on the opposite side of a curved element to a source of an acoustic signal.
 4. An apparatus according to claim 3, wherein the acoustic signal is the acoustic response to an electric pulse across a sample of a colloid.
 5. An apparatus for making pulsed acoustic measurements, including means for generating an acoustic response in a sample, a delay element, and a transducer for detecting the acoustic response, the transducer being positioned so as to receive said response through said delay element, characterised in that the delay element is shaped so that the walls of the delay element are orientated so that sidewall reflections do not substantially propagate to the transducer.
 6. An apparatus according to claim 5, wherein the delay element includes interfaces with a curved geometry, so as to deflect the sidewall reflections away from the site of the transducer.
 7. An apparatus according to claim 6, wherein the transducer is located on the opposite side of a curved element to a source of an acoustic signal.
 8. An apparatus according to claim 7, wherein the acoustic signal is the acoustic response to an electric pulse across a sample of a colloid.
 9. A method for making pulsed acoustic measurements, including the steps of: generating an acoustic response in a sample; providing a delay element through which an acoustic signal arising from the acoustic response propagates; and providing a transducer which detects the acoustic signal propagated from the delay element; characterised by arranging the delay element and transducer into a geometry which reduces the effect of sidewall reflections when compared to a similarly sized rectangular block delay element.
 10. A method according to claim 9, wherein the delay element includes interfaces with a curved geometry, so as to deflect the sidewall reflections away from the site of the transducer.
 11. A method according to claim 10, wherein the transducer is provided on the opposite side of a curved element to the source of the acoustic signal.
 12. A method according to claim 11, wherein the acoustic signal is the acoustic response to an electric pulse across a sample of a colloid.
 13. A method for making pulsed acoustic measurements, including the steps of: generating an acoustic response in a sample; providing a delay element through which an acoustic signal arising from the acoustic response propagates; and providing a transducer which detects the acoustic signal propagated from the delay element; characterised by providing a delay element having a shape, wherein the walls of the delay element are orientated so that sidewall reflections do not substantially propagate to the transducer.
 14. A method according to claim 13, wherein the delay element includes interfaces with a curved geometry, so as to deflect the sidewall reflections away from the site of the transducer.
 15. A method according to claim 14, wherein the transducer is provided on the opposite side of a curved element to the source of the acoustic signal.
 16. A method according to claim 15, wherein the acoustic signal is the acoustic response to an electric pulse across a sample of a colloid. 